The invention will be described primarily in connection with using light to obtain image data representing surface reflectivity of the external surfaces of boards of sawn timber in order to enable the computing of accurate image data of the three-dimensional surface profile of each individual board, for the purpose of adjusting sawing equipment in saw mills. This is important in a world of diminishing resources to optimize the quantity or value of the lumber produced. Image data is assembled from a sequence of surface scans of a board as it moves past a linear sensor of scanning apparatus. A typical scan would record image data 2048 pixels long by 1 pixel wide. However, area cameras could be used and larger swaths of pixel data would accordingly be input for image processing. Having computed the image of a board from scan data, algorithms can be applied that decide on the optimal placement of cuts by automated equipment in order to achieve desired characteristics of boards with minimal waste pieces. The surface features of the board captured in the image data disclose irregularities such as knots to be avoided or placed in order to meet the criteria for pieces to be made from the board. However, the invention is also applicable to measurement of other objects where rapid and accurate image capture may be beneficial.
The state of the art in target object imaging for industrial processing has been the obtaining of geometric, dimensional information from which a computer model of the object is constructed as if the object were homogeneous in composition.
The simplest non-contact automatic method commonly used to determine the shapes of boards is known in the prior art as shadow scanning. The board moves past a row of beams of light and the cross-sectional width of the board is determined by measuring the shadow cast by the board on an array of sensors on the other side of the board, which sensors are lined up with the projected light beams. Beams of light must be applied from several directions and sensed by a corresponding set of sensor arrays to obtain even a rough profile. The shadow method cannot measure or even detect concave features such as hole in the board. It measures the outer envelope of the profile of the board.
Other methods known in the prior art for determining the shape of an object without contact depend on the principle of triangulation, which has been known historically prior to the present century. The application of this principle can be illustrated by considering a single beam of light transmitted in a known direction in space from a known location at an object being measured. Some suitably selected form of receiving system positioned so as to view the object from a direction different from the direction at which the light was transmitted detects the direction from the receiving system at which the reflection from the projected light spot appears on the object being measured. The distance between the transmitter and the receiver is known and fixed. Hence two angles (determined from the transmitting and receiving directions) and one side of a triangle (the distance between the transmitter and the receiver) are determined, and thus the location of the spot on the object relative to the measuring apparatus is easily calculated. Triangulation is generally used to obtain geometric views and cannot by itself provide images of surface appearance variations that are not correlated with changes in geometric shape of the target object.
The present invention now provides a method and means for capturing enhanced surface appearance data and adding it to the geometric image of a target object.
Many industrial scanning applications require fast image capture (digital pictures) of target surfaces. All physical targets reflect incident light that falls on a surface in one of two kinds of reflection: specular reflection or diffuse reflection. Geometric imaging, the measuring and calculating from a distance of the profile of target objects having irregularities of shape moving rapidly along a production line, is plagued by instances of specular reflection of the light from the illumination source by various areas on the object to be imaged. Areas of specular reflection from the target object appear as overly bright areas on camera images and also obliterate image accuracy regarding surface appearance characteristics quite apart from variation in surface shape.
Specular reflection-is the mirror-like reflection of light (or sometimes other kinds of wave) from a surface, in which light from a single incoming direction (a ray) is reflected into a single outgoing direction. Specular reflection results from the tendency for incident light to be reflected at the same angle as the incidence angle on the opposite side of a normal to the surface. A mirror is an example of a very good specular reflector. Diffuse reflection is the tendency for incident light to be reflected in an omni-directional manner above the target surface. An example of specular vs. diffuse reflection can be found in comparison of “glossy” vs. “flat” paints—glossy painted surface is much more specularly reflective when compared with a surface painted with flat paint.
High speed image capture systems, used to scan dynamic scenes, benefit from a high intensity illumination source because camera exposure and integration time can then be reduced, enabling less smearing of the captured image and faster scan rates. This is particularly significant in industrial machine vision applications, when 2-dimensional images are obtained by combining a plurality of sequentially acquired linear scans. Machine vision is not restricted to 2 dimensional images generated from a plurality of sequentially acquired linear scans.
High quality image capture is desired or required in various machine vision applications to allow image processing to identify, isolate and classify features of interest in the image. Some aspects of image quality are predictable intensity response, ability to merge images captured from adjacent but similar image capture systems, with minimum “stitching” features which may negatively affect image processing. A good quality image having such characteristics can be obtained in an image acquisition system when only diffuse reflection—as opposed to specular reflection—from the target object is included in the image.
A classic challenge with image capture systems is the illumination system. Generally it is undesirable to have point-source lighting and desirable to have “flat” or “soft” lighting, that is, diffuse lighting. Non-diffuse lighting can result in peculiarities of contrast and shadow on images of the target object due to the light source's position. A source of light can be considered effectively a point source if the resolution of the imaging instrument is too low to resolve its size, or if the object is at a very great distance. To avoid hot spots created by specular reflection of one or of a few point source illuminators, many image capture illumination systems employ a large plurality of light sources and/or diffusing elements to try to minimize hot spots created by the specular reflectivity.
With a high speed moving target, the illuminator should be a flash rather than sustained ambient light, in order to capture the required image data for the system.
Historically, visual features of a board are only considered after cutting, at a sorting stage. The present invention enables the moving of such decisions upstream in the lumber milling process, and enables more usable and higher value end product than the prior technology.